1. Field of the Invention
This invention relates to a heat-resistant adhesive especially suitable for the bonding of a semiconductor chip to a lead frame and an adhesive member using the adhesive.
More particularly, this invention relates to a heat-resistant adhesive suitable for use in a semiconductor package, especially a package of LOC (lead on chip) structure, which is capable of preventing or minimizing occurrence of cracks when solder is subjected to reflow in conditions moisture is absorbed.
2. Description of the Related Art
The semiconductor package is fabricated by connecting a lead frame to a semiconductor chip using an adhesive or resonance solder and electrically connecting electrodes of the chip to the lead frame, followed by sealing the entire structure into a molded form as illustrated in FIGS. 1 thru 3.
In general, a conventional package has a structure as shown in FIG. 3 where the chip is mounted on a tab (die pad) of the lead frame. The connection between the lead frame and the chip is made by Au-Si resonance solder or a thermosetting epoxy adhesive (a die bonding agent).
However, as an integration degree in the package has become higher and a size of a chip to be mounted therein has become larger, the percentage of the volume occupied by the chip has become higher to such an extent that the chip cannot completely be encased in the structure as shown in FIG. 3 where the chip is mounted on the tab of the lead frame. To solve this problem, such a package having no tab as shown in FIG. 1 or FIG. 2 has been proposed, for example, in U.S. Pat. No. 5,140,404, Japanese Patent Application Publications (KOKAI) 61-218139 and 61-241959, U.S. Pat. No. 4,862,245, and Japanese Patent Application Publications (KOKAI) 2-36542 and 4-318962. The package structures of FIGS. 1 and 2 are called LOC (lead on chip) and COL (chip on lead), respectively. In these packages, the connection between the lead frame and the chip is made by a thermosetting adhesive or a heat-resistant hot melt.
With the structures as shown in FIGS. 1 and 2, it is a trend that the package size is becoming smaller and thinner and the percentage of the occupation by the chip is becoming higher, the thickness of the sealing material is becoming thinner accordingly. Therefore, moisture absorbed by the adhesive or the sealing material used would more often cause cracks in the package when it evaporates or expands.
To solve this problem, some attempts to improve the sealing materials and/or the adhesives employable for the package have been proposed. For example, with respect to the sealing materials, lowering of moisture absorption and improvement of mechanical strength have been discussed (Japanese Patent Application Publication (KOKAI) 5-67703). With respect to the adhesives, it has been proposed to reduce the moisture absorption or to divide an adhesive member into small pieces to promote escape of vapor during reflow in the soldering (Japanese Patent Application Publication (KOKAI) 3-109757). However, there has been no proposal about the approach from the properties of the adhesive to be employed for the LOC package.
There has been another attempt which intentionally allows a solvent to remain with a view to lowering a bonding temperature, lowering the viscosity of the resin (Japanese Patent Application Publication (KOKAI) 3-64386) or allows a solvent to remain to improve the fluidity and increase the adhesion for lowering a possible leak current between inner leads (Japanese Patent Application Publication (KOKAI) 2-36542).
It is therefore an object of the present invention to provide an adhesive suitable for use in a semiconductor package, especially a semiconductor package of LOC structure, which is capable of preventing possible cracks from occurring during reflow of solder under the conditions the package has absorbed moisture.
The inventors of the present invention has unexpectedly found, as a result of their research about a relationship between cracks developed in the LOC package during reflow of the solder under the conditions that the package has absorbed moisture and the properties of the adhesive employed, that the fluidity or hardness of the adhesive at a high temperature is more attributable to the package cracks than the moisture absorption or the glass transition point Tg of the adhesive. More particularly, they have found that an adhesive having a specific hardness or fluidity may advantageously be employed to prevent the package cracks in the LOC package. Thus, the present invention has been achieved.
Heretofore, there have only been known about the hardness of the adhesive that the solvent may be intentionally left with a view to lowering the bonding temperature to lower the viscosity of the resin (KOKAI 3-64386) or the solvent may be left to enhance the fluidity and improve the adhesion for reducing leak current between the inner leads (KOKAI 2-36542). It has not been known before the present invention that the hardness or retardation to flow of the adhesive is more attributable to the package cracks than the moisture absorption or Tg of the adhesion and that the adhesive having the specific hardness or fluidity is surprisingly effective to improve the resistance against the package cracks.
The present invention therefore provides:
(a) a heat-resistant adhesive suitable for use in an adhesive member for the fabrication of a semiconductor package by bonding a semiconductor chip to a lead frame with the adhesive member and molding at least the semiconductor chip and a bonded part between the semiconductor chip and the lead frame with a molding compound, in which the adhesive has a coming-out-length of not more than 2 mm and a water absorption rate of not more than 3 wt. %,
(b) a heat-resistant adhesive suitable for use in an adhesive member for the fabrication of a semiconductor package by bonding a semiconductor chip to a lead frame with the adhesive member and molding at least the semiconductor chip and a bonded part between the semiconductor chip and the lead frame with a molding compound, in which the adhesive has a coming-out length of not more than 2 mm, a water absorption rate of not more than 3 wt. % and a glass transition temperature of at least 200xc2x0 C.,
(c) a heat-resistant adhesive, in which the adhesive member is a composite adhesive sheet comprising a heat-resistant film and the heat-resistant adhesive applied in the form of a coating layer on one surface or opposite surfaces of the heat-resistant film, and
(d) a heat-resistant adhesive, in which the adhesive member consists of the heat-resistant adhesive alone.
The heat-resistant adhesives according to the present invention have excellent package-crack resistance and are effective especially for the improvement of the reliability of semiconductor packages.
The term xe2x80x9cwater absorption ratexe2x80x9d used herein means a water absorption rate obtained from a change in weight of an adhesive before and after immersion in water. Here, a film of the adhesive having a size of 5 cmxc3x975 cm with a thickness of 25 xcexcm is used as a testing object and, after dried at 100xc2x0 C. for 1 hour, it is immersed in water at 23xc2x0 C. for 24 hours to measure the water absoprtion.